3.2.2.1 Background

Higher plants acquire CO2 by diffusion through tiny pores (stomata)
into leaves and thus to the sites of photosynthesis. The total amount of CO2
that dissolves in leaf water amounts to about 270 PgC/yr, i.e., more than one-third
of all the CO2 in the atmosphere (Farquhar et al., 1993; Ciais et
al., 1997). This quantity is measurable because this CO2 has time
to exchange oxygen atoms with the leaf water and is imprinted with the corresponding
18O “signature” (Francey and Tans, 1987; Farquhar et al.,
1993). Most of this CO2 diffuses out again without participating
in photosynthesis. The amount that is “fixed” from the atmosphere,
i.e., converted from CO2 to carbohydrate during photosynthesis, is
known as gross primary production (GPP). Terrestrial GPP has been estimated
as about 120 PgC/yr based on 18O measurements of atmospheric CO2
(Ciais et al., 1997). This is also the approximate value necessary to support
observed plant growth, assuming that about half of GPP is incorporated into
new plant tissues such as leaves, roots and wood, and the other half is converted
back to atmospheric CO2 by autotrophic respiration (respiration by
plant tissues) (Lloyd and Farquhar, 1996; Waring et al., 1998).

Annual plant growth is the difference between photo-synthesis and autotrophic
respiration, and is referred to as net primary production (NPP). NPP has been
measured in all major ecosystem types by sequential harvesting or by measuring
plant biomass (Hall et al., 1993). Global terrestrial NPP has been estimated
at about 60 PgC/yr through integration of field measurements (Table
3.2) (Atjay et al., 1979; Saugier and Roy, 2001). Estimates from remote
sensing and atmospheric CO2 data (Ruimy et al., 1994; Knorr and Heimann,
1995) concur with this value, although there are large uncertainties in all
methods. Eventually, virtually all of the carbon fixed in NPP is returned to
the atmospheric CO2 pool through two processes: heterotrophic respiration
(Rh) by decomposers (bacteria and fungi feeding on dead tissue and exudates)
and herbivores; and combustion in natural or human-set fires (Figure
3.1d).

a WBGU (1988): forest
data from Dixon et al. (1994); other data from Atjay et al. (1979).b MRS: Mooney, Roy and Saugier (MRS) (2001). Temperate
grassland and Mediterranean shrubland categories combined.c IGBP-DIS (International Geosphere-Biosphere Programme
– Data Information Service) soil carbon layer (Carter and Scholes,
2000) overlaid with De Fries et al. (1999) current vegetation map
to give average ecosystem soil carbon.d WBGU boreal forest vegetation estimate is likely to
be to high, due to high Russian forest density estimates including standing
dead biomass.e MRS temperate forest estimate is likely to be too high,
being based on mature stand density.f Soil carbon values are for the top 1 m, although stores
are also high below this depth in peatlands and tropical forests.g Variations in classification of ecosystems can lead
to inconsistencies. In particular, wetlands are not recognised in the MRS
classification.h Total land area of 14.93x109 ha in
MRS includes 1.55x109 ha ice cover not listed in this table.
InWBGU, ice is included in deserts and semi-deserts category.

Most dead biomass enters the detritus and soil organic matter pools where it
is respired at a rate that depends on the chemical composition of the dead tissues
and on environmental conditions (for example, low temperatures, dry conditions
and flooding slow down decomposition). Conceptually, several soil carbon pools
are distinguished. Detritus and microbial biomass have a short turnover time
(<10 yr). Modified soil organic carbon has decadal to centennial turnover
time. Inert (stable or recalcitrant) soil organic carbon is composed of molecules
more or less resistant to further decomposition. A very small fraction of soil
organic matter, and a small fraction of burnt biomass, are converted into inert
forms (Schlesinger, 1990; Kuhlbusch et al., 1996). Natural processes and management
regimes may reduce or increase the amount of carbon stored in pools with turnover
times on the order of tens to hundreds of years (living wood, wood products
and modified soil organic matter) and thus influence the time evolution of atmospheric
CO2 over the century.

The difference between NPP and Rh determines how much carbon is lost or gained
by the ecosystem in the absence of disturbances that remove carbon from the
ecosystem (such as harvest or fire). This carbon balance, or net ecosystem production
(NEP), can be estimated from changes in carbon stocks, or by measuring the fluxes
of CO2 between patches of land and the atmosphere (see Box
3.1). Annual NEP flux measurements are in the range 0.7 to 5.9 MgC/ha/yr
for tropical forests and 0.8 to 7.0 MgC/ha/yr for temperate forests; boreal
forests can reach up to 2.5 MgC/ha/yr although they have been shown to be carbon-neutral
or to release carbon in warm and/or cloudy years (Valentini et al., 2000). Integration
of these and other results leads to an estimated global NEP of about 10 PgC/yr,
although this is likely to be an overestimate because of the current biased
distribution of flux measuring sites (Bolin et al., 2000).

When other losses of carbon are accounted for, including fires, harvesting/removals
(eventually combusted or decomposed), erosion and export of dissolved or suspended
organic carbon (DOC) by rivers to the oceans (Schlesinger and Melack, 1981;
Sarmiento and Sundquist; 1992), what remains is the net biome production (NBP),
i.e., the carbon accumulated by the terrestrial biosphere (Schulze and Heimann,
1998). This is what the atmosphere ultimately “sees” as the net land
uptake on a global scale over periods of a year or more. NBP is estimated in
this chapter to have averaged -0.2 ± 0.7 PgC/yr during the 1980s and
-1.4 ± 0.7 PgC/yr during the 1990s, based on atmospheric measurements
of CO2 and O2 (Section 3.5.1
and Table 3.1).

By definition, for an ecosystem in steady state, Rh and other carbon losses
would just balance NPP, and NBP would be zero. In reality, human activities,
natural disturbances and climate variability alter NPP and Rh, causing transient
changes in the terrestrial carbon pool and thus non-zero NBP. If the rate of
carbon input (NPP) changes, the rate of carbon output (Rh) also changes, in
proportion to the altered carbon content; but there is a time lag between changes
in NPP and changes in the slower responding carbon pools. For a step increase
in NPP, NBP is expected to increase at first but to relax towards zero over
a period of years to decades as the respiring pool “catches up”. The
globally averaged lag required for Rh to catch up with a change in NPP has been
estimated to be of the order of 10 to 30 years (Raich and Schlesinger, 1992).
A continuous increase in NPP is expected to produce a sustained positive NBP,
so long as NPP is still increasing, so that the increased terrestrial carbon
has not been processed through the respiring carbon pools (Taylor and Lloyd,
1992; Friedlingstein et al., 1995a; Thompson et al., 1996; Kicklighter et al.,
1999), and provided that the increase is not outweighed by compensating increases
in mortality or disturbance.

The terrestrial system is currently acting as a global sink for carbon (Table
3.1) despite large releases of carbon due to deforestation in some regions.
Likely mechanisms for the sink are known, but their relative contribution is
uncertain. Natural climate variability and disturbance regimes (including fire
and herbivory) affect NBP through their impacts on NPP, allocation to long-
versus short-lived tissues, chemical and physical properties of litter, stocks
of living biomass, stocks of detritus and soil carbon, environmental controls
on decomposition and rates of biomass removal. Human impacts occur through changes
in land use and land management, and through indirect mechanisms including climate
change, and fertilisation due to elevated CO2 and deposition of nutrients
(most importantly, reactive nitrogen). These mechanisms are discussed individually
in the following sections.